U.S. patent application number 11/652250 was filed with the patent office on 2007-06-28 for magnetically shielded container.
Invention is credited to Elke Aidam, Michael Ebert, Tino Grossmann, Werner Heil, Ernst-Wilhelm Otten, Daniela Rohe, Reinhard Surkau.
Application Number | 20070145305 11/652250 |
Document ID | / |
Family ID | 7843731 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070145305 |
Kind Code |
A1 |
Aidam; Elke ; et
al. |
June 28, 2007 |
Magnetically shielded container
Abstract
A gas storage cell containing a nuclear spin polarized gas in a
gas storage space surrounded by a cell wall made of an uncoated
material having a surface substantially free of paramagnetic
substances which is in contact with the gas storage space and a
method for removing the nuclear spin polarized gas from the gas
storage cell.
Inventors: |
Aidam; Elke; (Worth, DE)
; Ebert; Michael; (Worrstadt, DE) ; Grossmann;
Tino; (Mainz, DE) ; Heil; Werner; (Oppenheim,
DE) ; Otten; Ernst-Wilhelm; (Mainz, DE) ;
Rohe; Daniela; (Basel, CH) ; Surkau; Reinhard;
(Mainz, DE) |
Correspondence
Address: |
WATOV & KIPNES, P.C.
P.O. BOX 247
PRINCETON JUNCTION
NJ
08550
US
|
Family ID: |
7843731 |
Appl. No.: |
11/652250 |
Filed: |
January 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09509317 |
Feb 2, 2001 |
7176386 |
|
|
11652250 |
Jan 11, 2007 |
|
|
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Current U.S.
Class: |
250/515.1 ;
206/719 |
Current CPC
Class: |
G21F 5/00 20130101 |
Class at
Publication: |
250/515.1 ;
206/719 |
International
Class: |
G21F 3/00 20060101
G21F003/00; G21F 1/00 20060101 G21F001/00; B65D 85/00 20060101
B65D085/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 1997 |
DE |
19742548.8 |
Sep 24, 1998 |
EP |
PCT/EP98/06056 |
Claims
1-44. (canceled)
45. A gas storage cell (20) containing a nuclear spin polarized gas
in a gas storage space surrounded by a cell wall, the wall being of
an uncoated material which on the surface contacting said gas
storage space is substantially free of paramagnetic substances.
46. A cell as claimed in claim 45 wherein said wall is of a low
iron content glass, the iron content being sufficiently low that
the ratio between the wall-related depolarization relaxation time
T.sub.1.sup.w for nuclear spin polarized .sup.3He and the
volume-to-inner surface area of said cell is at least 10
hours/cm.
47. (canceled)
48. A method for the removal of a nuclear spin polarized gas from a
gas storage cell (20) in a magnetically shielded container (1);
said container having, disposed in parallel opposed position on an
axis (S) thereof, magnetic field homogenizing pole shoes (10.1,
10.2); having disposed about said pole shoes a magnetically
shielding yoke (2), said pole shoes and yoke enclosing a magnetic
chamber (26); said container further comprising magnetic field
sources (2.4, 2.5) disposed about and radially distanced from said
axis, whereby there exists within said chamber substantially
homogeneous magnetic field B.sub.o oriented in the direction of
said axis, and whereby there is a usable volume within said chamber
where the ratio of the magnetic field gradient in the direction
transverse to said axis to said magnetic field B.sub.o has a value
of no more than 1.5.times.10.sup.-3/cm, the method comprising: (i)
positioning said container with said axis (S) parallel to the field
direction of an external substantially homogeneous magnetic field;
(ii) opening said container by removing a portion comprising one of
said pole shoes (10.1); and (iii) removing said cell (20) in the
direction of said axis.
Description
[0001] This application is a divisional application of U.S. patent
application Ser. No. 09/509,317 filed on Sep. 24, 1998.
[0002] The invention relates to a magnetically shielded container,
e.g. usable as a transport device for spin polarized gases, and to
a storage cell useful therein.
[0003] Nuclear spin polarized gases, in particular noble gases such
as the helium isotope with the mass number 3 (.sup.3He) or the
xenon isotope with the mass number 129 (.sup.129Xe) and gases
containing the fluorine, carbon or phosphorus isotopes .sup.19F,
.sup.13C or .sup.31P are required for a great number of experiments
in fundamental physics research. In the field of medicine, such
isotopes are, in particular, considered for use in nuclear magnetic
resonance imaging, of the lungs for example. (See for example WO
97/37239, WO 95/27438, Bachert et al., Mag Res Med 36: 192-196
(1996) and Ebert et al., The Lancet 347: 1297-1299 (1996)). A
prerequisite for the use of such spin polarized gases in nuclear
magnetic resonance imaging is that the degree of polarization P of
the spin I of the nuclei, or the associated magnetic dipole moment
.mu..sub.I, is greater by an order of 4-5 than is normally achieved
in thermal equilibrium in the magnetic field B.sub.T of the
magnetic resonance imaging apparatus. This normal degree of
polarization, P.sub.Boltzmann, is dependent on the magnetic dipole
energy -.mu..sub.IB.sub.T and average thermal energy kT:
P.sub.Boltzmann=tan h(.mu..sub.IB.sub.T/kT) (1) (where
k=Boltzmann's constant, and T=absolute temperature). Where
P.sub.Boltzmann<<1, then it approximates to
.mu..sub.IB.sub.T/kT.
[0004] Whereas the hydrogen isotope .sup.1H used in magnetic
resonance imaging of tissues only reaches a P.sub.Boltzmann of
5.times.10.sup.-6 at B.sub.T=1.5 T and T=300 K, a
P.gtoreq.1.times.10.sup.-2, i.e. 1%, is required in gas magnetic
resonance imaging. The requirement for such an extremely increased
P primarily results from the low concentration of the gas atoms in
comparison with that of the hydrogen in the tissue. Gases with such
degrees of polarization (normally referred to as hyperpolarized
gases) can be produced by means of various known methods,
preferably optical pumping.
[0005] In addition, for gas magnetic resonance imaging relatively
large quantities of gas, of the volume of a breath for example (0.5
to 1 litre), are needed.
[0006] Particularly high degrees of polarization, for example
>30%, combined with high rates of production, e.g. 0.5 litres/h,
may be achieved through compression of an optically-pumped gas.
This process is described in the following publications, the
content of which is incorporated herein by reference:
[0007] Eckert et al., Nuclear Instruments and Methods in Physics
Research A 320: 53-65 (1992);
[0008] Becker et al., J. Neutron Research 5; 1-10 (1996);
[0009] Surkau et al., Nuclear Instruments and Methods in Physics
Research A 384: 444-450 (1997);
[0010] [Neil] Heil et al., Physics Letters A 201: 337-343
(1995).
[0011] However production and use of hyperpolarized gases do not
necessarily occur at the same site and the problem thus arises of
transporting the polarized gases, produced for example using the
method described above, to the consumer, for example for use in a
nuclear magnetic resonance imaging apparatus for the lungs.
Previously, transportable magnetic devices which provide a
sufficiently homogeneous magnetic holding field for a large storage
volume of such a spin polarized gas were not available.
Furthermore, the nuclear spins very rapidly depolarized on the cell
walls, so that polarized gases could only be stored for a short
time while retaining the necessary degree of polarization.
[0012] One problem addressed by the invention is to provide a
magnetic device capable of providing a transportable, homogeneous
magnetic holding field for a sufficiently large storage volume of
hyperpolarized gas.
[0013] Viewed from one aspect the invention thus provides a
magnetically shielded container having disposed in parallel opposed
position on an axis thereof magnetic field homogenizing pole shoes,
having disposed about said pole shoes a magnetically shielding
yoke, said pole shoes and yoke enclosing a magnetic chamber, said
container further comprising magnetic field sources disposed about
and radially distanced from said axis whereby there exists within
said chamber a substantially homogeneous magnetic field B.sub.o
oriented in the direction of said axis and whereby there is a
usable volume within said chamber where the ratio of the magnetic
field gradient in the direction transverse to said axis to said
magnetic field B.sub.o has a value of no more than
1.5.times.10.sup.-3/cm.
[0014] Such a container may be constructed in a form which is low
in weight, simple in structure, and inexpensive to manufacture and
economical in use. Furthermore, using the container, nuclei which
are transported can, as far as possible, retain their orientation,
even in external stray fields, i.e. the depolarization relaxation
times may be as long as possible in order to prevent a
disorientation of the nuclear spin of the gas.
[0015] The container of the invention, which is suitable for
containing and transporting spin polarized atoms, especially
polarized .sup.3He and .sup.129Xe, is preferably provided with
magnetic field homogenising, highly-permeable and magnetically soft
plates, e.g. of .mu.-metal or soft iron, as pole shoes, and is so
structured that a very large ratio can be achieved between the
usable volume, within which a sufficiently homogeneous magnetic
field is present, and the total volume, e.g. a ratio of at least
1:30. However, this ratio is preferably at least 1:5, more
preferably 1:3 and, particularly advantageously 1:2. A ratio of
1:1.5 can be achieved. A value of
G.sub.r=((.delta.B.sub.r/.delta.r)/B.sub.o).ltoreq.1.5.times.10.sup.-3/cm
(2) is hereby applied as a homogeneity condition within the usable
volume for the relative transverse gradient G.sub.r of the magnetic
field B.sub.o. This requirement results from the gradient-dependent
relaxation time T.sub.1G, which (at high pressures, such as those
the present invention is concerned with) is related as follows to
G.sub.r and the gas pressure p:
T.sub.1G=p/G.sub.r.sup.2.times.(1.75.times.10.sup.4
cm.sup.2bar/h).sup.-1 (3) (see Scherer et al., Phys Rev 139: 1398
(1965)).
[0016] According to equation (3), with
G.sub.r<1.3.times.10.sup.-3/cm and p=3 bars, a
gradient-dependent relaxation time T.sub.1G>76 h is
achieved.
[0017] At lower pressures,
T.sub.1G=p/G.sub.r.sup.2.times.(1.8.times.10.sup.3 cm.sup.2
bar/h).sup.-1 (see Barbe, Journal de Physique 35: 699 and 937
(1974)).
[0018] During the movement of a polarized gas storage cell into the
container of the invention, G.sub.r will generally be less than
0.02.times.10.sup.-3/cm. In this way .sup.3He at 3 bar loses only
2% polarization per 30 seconds.
[0019] Within the container according to the invention, G.sub.r is
preferably no more than 1.3.times.10.sup.-3/cm, more preferably no
more than 7.times.10-4/cm. With a gas storage cell radius of 8 cm,
G.sub.r of .ltoreq.1.3.times.10.sup.-3/cm corresponds to T.sub.1G
of .gtoreq.127 hours, while with a gas storage cell radius of 2 cm,
G.sub.r of .ltoreq.7.times.10.sup.-4/cm corresponds to T.sub.1G of
.gtoreq.350 hours.
[0020] In order to compensate field distortions in the marginal
areas of the interior space of the container and thus improve the
homogeneity of the magnetic field B.sub.o, the container features
magnetic field sources which are arranged in such a way that the
field distortions in the marginal areas of the interior space of
the container are minimal and the field in the interior of the
container is largely homogeneous.
[0021] In order to maintain the polarization of the nuclear spin
once it has been achieved, only a relatively weak homogeneous
magnetic field is required which preferably displays a magnetic
field strength of less than 5 mT, more preferably less than 1 mT,
more especially in the range 0.2 to 0.9 mT. In such a weak magnetic
field, continuous quality control of the degree of polarization can
be achieved with the aid of measuring instruments, ensuring
particular reliability. Thus in one preferred embodiment, a
magnetic field sensor (e.g. one based on the Forster principle) is
disposed in the container of the invention so as to allow
determination of the magnetic field B.sub.d generated by the
hyperpolarized gas.
[0022] Whereas the generation of strictly homogeneous magnetic
fields with the aid of ferromagnetic materials previously
concentrated on high field strengths within the tesla range, the
concept behind the container of the invention is deliberately
focused on the most efficient and practical realisation of a weak,
widely homogeneous magnetic field, e.g. using ferromagnetic
materials.
[0023] A high degree of homogeneity can be achieved within the weak
field range if, for example, as homogenising ferromagnetic
elements, two thin soft iron, or more preferably .mu.-metal, plates
are used as pole shoes. Such pole shoes, thanks to their extremely
high permeability and low remanence, create a very homogeneous
field within the intervening space, the magnetic chamber.
[0024] In a particularly preferred embodiment, the homogenising
effect of these pole shoes can be increased by introducing magnetic
resistances between the pole shoes and the yoke. A preferred
material for a magnetic resistance of this sort, is a rigid
non-magnetic layer, for instance in the form of a plate, for
example of plastic, fitted between the pole shoe and the yoke. If
such a plate or, in order to save weight, preferably[,] a porous,
e.g. honeycomb structure, is also bonded to the pole shoe, this
guarantees its flatness which allows the pole shoes to be parallel
and the field B.sub.o to be homogeneous.
[0025] In order to fulfil the aforementioned homogeneity conditions
in the simplest possible manner, and at the same time to provide a
large storage volume, it has proved especially preferable to design
the container of the invention in the form of a pot magnet. A
magnetic device of this sort consists essentially of a closed pot
which, in an exemplary construction form, can have a diameter of
30-60 cm with an overall height of 10-30 cm. The particular
advantage of designing the container in the form of a pot magnet
lies in the high degree of symmetry of this cylindrical
construction. Two possibilities can be considered as particularly
preferred arrangements of the field sources in a pot magnet of this
sort:
[0026] positioning the field sources, for example in the form of
commercially-available permanent magnetic plates, in a gap in the
median or reflection plane of the pot; and
[0027] positioning the field sources on the outer surface of the
end plates of the pot.
[0028] By appropriately dividing the field sources between these
two arrangements, on the one hand positioning the field sources in
the median plane, on the other hand positioning the field sources
on the outer surface of the end plates of the pot, it is possible
to correct the boundary errors of the magnetic field inside the pot
magnet and thus fulfil the homogeneity conditions over a wide range
in a radial direction. A preferred division is such that the
increase in the boundary field which occurs when the field sources
are arranged in the reflective or median plane of the pot magnet is
just compensated by the fall-off in the boundary field which occurs
where the field sources are positioned on the end plate of the
pot.
[0029] If desired, magnetic field sources may be placed elsewhere
in the container of the invention so as to achieve an improvement
in the homogenization of the applied field B.sub.o. Thus for
example such sources may be placed in further planes perpendicular
to B.sub.o besides the planes of, adjacent to and mid-way between
the pole shoes.
[0030] A particularly homogeneous boundary field is also achieved
if a magnetic screen, e.g. a soft iron or .mu.-metal ring, is
fitted between the pot and the rim of the pole shoe, so that an
external stray field is partially short-circuited and, where the
field sources are arranged on the median plane of the pot magnet,
the value of the boundary field is reduced to the value of the
central field in the centre of the pot magnet through appropriate
dimensioning of the magnetic screen.
[0031] Advantageously, especially in the case of non-circular
cylindrical (e.g hexagonal-cylindrical) containers according to the
invention, shims (e.g. corner shims-positioned onto the pole shoes)
may be used to improve field homogeneity within the magnetic
chamber. Advantageously also the chamber has a high degree of
azimuthal symmetry.
[0032] Two preferred construction forms can be used as magnetic
field sources. In a first construction form, permanent magnets can
be used, preferably commercially-available tablets, for example
with a height of 5 mm and a diameter of 20 mm. In another
construction form, these permanent magnets are replaced with
appropriately-dimensioned magnetic field coils. Such magnetic field
coils have the advantage that the desired magnetic fields can be
adjusted by means of an appropriately-selected current flow.
However, a disadvantage of the second construction form is that an
additional current source must be carried with the container where
it is used as a transport device rather than simply as a storage
device.
[0033] The container is advantageously constructed using a yoke of
a material which is not magnetically saturated at fields below 1
Tesla, more preferably 2 Tesla, e.g. a soft iron. The container
dimensions are preferably such that the usable volume (within which
the gas storage cell may be disposed) is at least [50 mL] 50 ml,
more preferably [100 mL] 100 ml, especially preferably [200 mL] 200
ml to greater than [1 m.sup.3] 1 l, e.g. up to [20 L] 20 l, more
particularly [200-2000 mL] 200-2000 ml. The materials used can
allow a total container weight to magnetic chamber volume of no
more than 1 [g/L] kg/l, more preferably [0.2 kg/L] 0.2 kg/l,
especially preferably [ 1/30 kg/L] 1/30 kg/l. The gas storage cell
which can be disposed in the container, e.g. for storage or
transport, preferably has an internal volume of at least [50 mL] 50
ml, e.g. [100 mL] 100 ml to [1 m.sup.3] 1 l, particularly [100 mL]
100 ml to [20 L] 20 l, more particularly [200 mL] 200 ml to [2 L] 2
l. This cell may be provided with a valve for allowing gas
introduction and removal; alternatively it may be a single-use
cell, e.g. provided with a sealable portion and a breakable portion
(which may be the sealable portion after sealing).
[0034] In one embodiment, the container of the invention may take
the form of a magnetic device with an internal space which provides
a high-volume, largely homogeneous, shielded magnetic field within
its interior, whereby the magnetic device features homogenising
.mu.-metal plates as pole shoes, the magnetic device is
characterised in that a ratio of 1:1.5 can be achieved between the
useable volume of the magnetic device within which a homogeneous
magnetic field is present and the overall volume of the magnetic
device and the homogeneity condition
G.sub.r.ltoreq.1.5.times.10.sup.-3/cm is fulfilled within the
useable volume, where G.sub.r is the relative transverse magnetic
field gradient.
[0035] Viewed from a further aspect, the invention also provides a
gas storage cell containing a nuclear spin polarized gas in a gas
storage space surrounded by a cell wall, the wall being of an
uncoated material which on the surface contacting said gas storage
space is substantially free of paramagnetic substances. The gas may
for example be .sup.3He or .sup.129Xe, especially .sup.3He. Using
an essentially paramagnetic substance free cell wall makes it
possible for polarized .sup.3He to display a wall-related
depolarization relaxation time T.sub.1.sup.w of at least 20 hours.
It is particularly preferable that the wall-related depolarization
relaxation time be more than 50 hours. Such high depolarization
relaxation times can be achieved if a material is used as cell wall
material which contains a low proportion of paramagnetic atoms or
molecules, whereby in a particularly preferred construction form
glasses with very low iron concentrations, preferably less than 20
ppm, are used, which can also be composed in such a way that, at
the same time, they represent an efficient diffusion barrier
against helium, for example Supremex glass (manufactured by Schott,
Mainz, Del.) of the type of the alumina silicate glasses. In
comparison with the previously-known storage cells described by
Heil et al. in Physics Letters A 201: 337-343 (1995), long
wall-related depolarization relaxation times can be achieved using
the storage cells in accordance with the invention, without complex
metal coating of the walls being necessary.
[0036] As mentioned above, the container of the invention may take
the form of a transport device for spin polarized gases, especially
.sup.3He and .sup.129Xe or gases containing .sup.19F, .sup.13C or
.sup.31P, e.g. gases which have been spin polarized by polarization
transfer. Within the area in the interior space of the container in
which the storage cell is positioned, the magnetic field of the
magnetic device can be so homogeneous that the depolarization
relaxation time T.sub.1.sup.g caused by a transverse magnetic field
gradient in accordance with equation (3) is greater than 125 hours,
especially greater than 200 hours, more particularly greater than
300 hours, preferably greater than 500 hours, particularly
preferably greater than 750 hours, and the wall-related
depolarization relaxation time T.sub.1.sup.w, due to impacts of the
nuclear-polarized gas on the wall of the storage cell, is greater
than 5 hours, preferably greater than 20 hours.
[0037] More preferably, T.sub.1.sup.w normalized by the interior
surface to volume ratio of the storage cell is preferably at least
10 h/cm.
[0038] However, depolarization losses occur not only during the
transport of the gas, due to the influence of external stray
magnetic fields and the resulting inhomogeneity of the magnetic
field, or due to collisions between the atoms and the wall, but, in
particular, also when the gas is removed from the transport
container.
[0039] Viewed from a still further aspect, the invention therefore
provides a method for the removal of a nuclear spin polarized gas
from a gas storage cell in a container comprising:
(i) positioning said container with [said] its axis parallel to the
field direction of an external substantially homogeneous magnetic
field;
(ii) opening said container by removing a portion comprising one of
said pole shoes; and
(iii) removing said cell in the direction of said axis.
[0040] Such depolarization losses can be minimised if the removal
of the polarized gas takes place according to this method.
[0041] In this method, the container, e.g. in the form of a pot
magnet, is set up with its axis and the alignment of the internal,
homogeneous magnetic field parallel to an external, adequately
homogeneous magnetic field, which can, for example, be achieved
with the aid of a Helmholz coil or the stray field of a nuclear
magnetic resonance imaging apparatus. The half of the pot magnet
facing the homogeneous magnetic field in an axial direction is then
lifted off. The remaining half then guarantees a sufficient field
homogeneity in the area of the gas cell through the magnetic
equipotential surface of its pole shoe, which is made, for
instance, of .mu.-metal. The removal of the storage cell filled
with polarized gas from the magnet can take place in an axial
direction within a few seconds.
[0042] Embodiments of the invention are described by way of
non-limiting examples, with reference to the accompanying drawings,
in which:
[0043] FIG. 1: shows an external perspective view of the container
of the invention;
[0044] FIG. 2: shows a cross section through a container in
accordance with the invention, which is in pot magnet form and
contains a storage cell for spin polarized gases positioned within
its interior;
[0045] FIGS. 3a-d: show various arrangements for boundary field
compensation;
[0046] FIG. 4: shows a further variant of the container in
accordance with the invention;
[0047] FIG. 5a: shows the curve of the value of the relative,
radial gradient G.sub.r in the radial direction R of a pot magnet
for different arrangements of the field sources;
[0048] FIG. 5b: shows the curve of FIG. 5a with the scale modified
for emphasis;
[0049] FIG. 6: shows the relaxation of .sup.3He polarization in a
storage cell made [of] of glass with a low iron content, whereby
the volume of the cell is, for example, 350 cm.sup.3 and the gas
pressure 2.5 bars;
[0050] FIGS. 7a-b: demonstrate the removal of a storage cell from a
container according to the invention placed within an external
field; and
[0051] FIG. 8: shows a further variant of a container according to
the invention which has non-circular cylindrical symmetry.
[0052] Referring to FIG. 1, there is shown an external perspective
view of a container 1 in accordance with the invention, which in
this instance is designed as a two-part cylindrical pot magnet with
an upper section 1.1 and a lower section 1.2. Also indicated [is]
are the rotationally symmetrical axis S of the pot magnet and the
magnetic field line of external magnetic fields, for example the
earth's magnetic field. Especially clearly shown is the path of an
external magnetic field or stray field B.sub.s.sup..perp. which
does not penetrate into the interior of the pot magnet but, due to
the slight magnetic resistance of the yoke 2, which is preferably
made of [a] soft iron material, is conducted around the interior
space. The stray field B.sub.s.sup.II is perpendicular to the
end-plates of the yoke and is homogenised by the .mu.-soft iron
pole shoes positioned inside the yoke 2.
[0053] FIG. 2 shows an axial cross section through a container for
spin polarized gases, especially .sup.3He, .sup.129Xe, as shown in
FIG. 1, comprising the container in accordance with the invention
and a storage cell for spin polarized gas positioned inside it,
which is characterised by extremely long wall depolarization
relaxation times.
[0054] The pot magnet 1 comprises a cylindrically-formed yoke 2,
preferably made of soft iron for returning the magnetic flux and
for shielding off external fields. In turn, the
cylindrically-formed yoke 2 features two yoke end plates forming a
central section 2.1. In the construction form shown, the yoke end
plates 2.1 take the form of two circular discs 2.1.1 and 2.1.2.
Closed surrounding sheets 2.2 and 2.3 are arranged around the rim
of the yoke end plates to form a yoke jacket. These differ in the
two construction forms shown in the left and right halves of FIG.
2. The surrounding sheets 2.2 and 2.3 are arranged both on the
upper disc 2.1.1 and also on the lower disc 2.1.2, resulting in an
upper section and a lower section of the pot magnet, which, in the
first construction form shown on the left, meet at the projecting
angled peripheral flanges 2.2.1 in the median plane of the magnetic
device. In the second construction form shown on the right, the
peripheral flanges 2.3.1 are spaced in such a way that an opening
for holding field sources, for example permanent magnets, is formed
in the median plane 4 of the pot magnet 1. The field line produced
due to the positioning of the field sources, for example the
permanent magnets, in the centre between the upper and lower
peripheral flanges of the pot magnet is identified with 6. In the
first construction form shown on the left, the height of the two
halves of the yoke jacket 2.2 exceeds the distance between the yoke
end plates 2.1.1, 2.1.2. It is possible to position field sources
on the outer surface 2.5 in the gap between the jacket and the end
plate. The field line in the boundary region which results with
such an arrangement is identified with the number 8.
[0055] The two opposing pole shoes 10.1 and 10.2 are responsible
for the homogeneous field within the interior of the pot magnet. In
this example, the pole shoes are essentially designed as
homogenising .mu.-metal plates. .mu.-metal is a material with a
very high homogenising force in relation to an external, stray
magnetic field B.sub.x.sup.II and is distinguished by very low
remanences.
[0056] In this example, .mu.-metal A manufactured by
Vacuumschmelze, P.O. Box 2253, 63412 Hanau with the following
magnetic characteristics is used: TABLE-US-00001 Stat. coercivity:
H.sub.c .ltoreq.30 mA/cm Permeability: .mu..sub.(4) .gtoreq.30,000
Maximum permeability: .mu..sub.(max) .gtoreq.70,000 Saturation
inductance: B.sub.2 .gtoreq.0.65 T
(This should not be interpreted as meaning that only this material
can be used for the invention). Over the entire pole shoes, the
distance between the shoes, and the parallel orientation of the
pole shoes may be ensured by the provision of spacer elements or
spacer rings, e.g. a total of three (or more) spacers 12, of which
only one is shown in FIG. 2.
[0057] The resulting homogeneous magnetic field between the pole
shoes 10.1 and 10.2, made of .mu.-metal, is identified with the
reference number 14 in this representation. As can be seen from the
representation in FIG. 1, a particularly homogeneous magnetic
field, independent of external fields, is achieved inside the pot
magnet due to the homogenising force of the .mu.-metal, whereas, in
the marginal areas, depending on the arrangement of the field
sources, a different field pattern 6 or 8 occurs. If the field
sources are arranged solely in the median plane 4, as shown for the
right-hand marginal area of the pot magnet 1, then a considerable
part of the magnetic flux escapes from the jacket due to the low
magnetic resistance and, acting from the edge, interferes with
field between the pole shoes, with an amplifying effect. The field
therefore increases significantly in intensity towards the edge, as
a result of which the desired homogeneity is impaired even where
the two pole shoes are a relative short distance apart. Where the
permanent magnets are positioned on the outer surface on the end
plates of the pot, as shown in FIG. 2 for the left-hand half of the
magnet, a significant marginal fall-off of the field is observed
between the pole shoes 10.1,10.2, as shown by the field line 8,
because the jacket, which reaches right up to the pole shoes,
attracts and weakens the boundary field.
[0058] The very homogeneous field 14 produced in the intervening
space due to the extremely high permeability of the .mu.-metal
plates used as pole shoes 10.1,10.2 can be increased even further
through the introduction of a magnetic resistance 16 between the
pole shoes 10.1, 10.2 and the yoke 2.1.1 and 2.1.2. A rigid,
non-magnetic plate, for example a plastic plate 16 or, in order to
save weight, preferably a honeycomb structure, is preferably used
for this purpose. The plate 16 can be bonded to the pole shoes
10.1,10.2, thus guaranteeing the flatness of the pole shoes
10.1,10.2.
[0059] The storage cell 20 for holding the polarized gas is located
in the central mid-section of the pot magnet 1 between the two pole
shoes 10.1,10.2. The container 20 is preferably manufactured of
iron-free glass and has an iron concentration of less than 20 ppm,
for example, and can also be designed in such a way that it also
forms an efficient diffusion barrier against helium. This measure
allows wall-related relaxation times of more than 70 hours to be
achieved. The storage cells 20 can be pumped out prior to use and,
for example, as is usual in high-vacuum technology, heated through
until their residual water layers are lost. This measure is
advantageous in the invention, but by no means necessary. The
storage cells are, for example, sealed with a glass stopcock 22 and
are connected to the filling unit for the polarized gas via a glass
flange 24.
[0060] In addition, in order to determine the degree of
polarization, a high-frequency coil 30 (which can be used to
subject the storage cell 20 to a time-variant magnetic field) and a
detection device (e.g. a magnetic field sensor) 32 can be fitted as
may means for moving sensor and storage cell relative to each
other. However, these additional fixtures are optional and are by
no means essential for a transport device in accordance with the
invention.
[0061] Furthermore, the container may if desired be fitted with
cooling means to cool the contents of the gas storage cell.
[0062] The decisive feature of the invention is that a magnetic
field is created within the container which is homogeneous over a
very large volume, so that a high usable volume is achieved in
relation to the total volume of the magnetic device, whereby the
homogeneous field within the interior of the magnetic device is
essentially not to be interfered with by external magnetic fields.
On the one hand, the low magnetic field strength of B.sub.o<1 mT
which may be used allows a very lightweight construction of the
yoke and pole shoes using thin soft iron sheeting. On the other
hand, it is desirable that the pole shoes display particularly low
remanence, so that these are therefore preferably made of
.mu.-metal in order to fulfil the homogeneity requirement (2).
[0063] In terms of being able to determine the degree of
polarization, it is advantageous if the homogeneous holding field
in the interior of the magnet is a weak magnetic field with a field
strength of less than 1.0 mT, since the magnetic fields caused by
the spin polarization of the gas, which lie within the nano to
micro Tesla range, can then still be measured with sufficient
accuracy with the aid of the simple detection device 32 and the
degree of polarization determined on this basis. This is
advantageous if, for example, the quality of the delivered gas has
to be tested prior to a medical application.
[0064] FIG. 3 shows the field distribution within the marginal area
achieved by means of different arrangements of field sources,
either alone or in combination with a magnetic screen, which
guarantees a sufficiently homogeneous field distribution within the
marginal area.
[0065] FIG. 3a shows an arrangement in which the permanent magnets
are placed inside the gap 2.4 and inside the gap 2.5 on the end
plates of the pot 2.1.1, 2.1.2. By dividing the arrangement of the
permanent magnets 2.4. appropriately between arrangement in the
centre 4 and arrangement on the end plates of the pot 2.1.1, 2.1.2,
the increase in the intensity of the boundary field 6, which is
caused by the positioning of the permanent magnets in the centre
between the end plates of the pot, as shown, is just compensated by
the fall-off in the intensity of the boundary field 8 of the
permanent magnets arranged on the end plates of the pot. If the
individual permanent magnets are of equal magnetic field strength,
an optimal distribution of the permanent magnets is achieved, for
the height-to-width ratio of the pot shown in the drawing, if the
magnets are distributed in a numerical ratio of 6:8, whereby the
first figure represents the number of magnets which are arranged in
the median plane 4, and the second figure represents the number of
magnets which are arranged on the end plates of the pot.
[0066] FIG. 3b shows a possible homogenisation of a boundary field
using permanent magnets arranged in the median plane 4 with the aid
of a magnetic screen 40. A magnetic screen of this sort is, for
example, formed by a soft iron ring which is introduced between the
pot and the rim of the pole shoe and which, like the sheets
2.2,2.3, runs around it. Such a soft iron ring partially
short-circuits the stray external field and, if appropriately
dimensioned, reduces the boundary field to the value of the central
field.
[0067] FIGS. 3c and 3d show means of compensation which are
comparable with those shown in FIGS. 3a and 3b where, in this
example, magnetic coils 50, 52 arranged centrally in the area of
the median plane 4 of the pot or in the vicinity of the end plates
of the pot are used as field sources instead of permanent
magnets.
[0068] FIG. 3c shows the compensation achieved through a suitable
ratio of field sources arranged in the median plane to field
sources arranged in the vicinity of the end plates of the pot, and
FIG. 3d shows the compensation with the aid of a magnetic screen
40.
[0069] A further construction form of the invention is shown in
FIG. 4. In order to reduce weight, the yoke jacket is constructed
of very thin surrounding sheets 200.1,200.2 and 202.1 and 202.2, in
a double-walled construction. The surrounding sheets 200.1, 200.2
and 202.1 and 202.2 are arranged at a fixed distance from one
another using spacing rings 207, so that a double shielding of the
interior of the pot magnet 1 is achieved. These can be considerably
thinner than in a single-walled construction form as shown in FIG.
1, while displaying the same capacity to conduct magnetic fluxes
away via the shielding rings. The surrounding sheets are connected
with the upper or lower .mu.-metal plate of the pot magnet via a
screwed connection 204 or 206. The pole shoes 10.1 and 10.2 are
spaced apart by means of spacing elements or a spacing ring 205
which may be circular or polygonal, e.g. hexagonal, in
cross-section. The homogeneous magnetic field is essentially formed
in the interior 208 between the pole shoes. As in FIG. 3a, the
permanent magnets 210 fitted in the gap 2.4 between the upper and
lower section of the pot magnet and between the jacket and end
plate serve as sources for a field which is also homogeneous in the
marginal area.
[0070] FIGS. 5a and 5b show the curve of the amount of the
relative, radial gradient
G.sub.r=((.delta.B.sub.r/.delta..sub.r)/B.sub.o) measured 1.5 cm
above the reflective plane 4 of the pot magnet in a radial
direction r for different arrangements of the permanent magnets in
or on the pot magnet in accordance with the invention. The curve
marked "a" shows the curve produced when permanent magnets are only
arranged in the gap in the median plane 4, as shown in the right
half of FIG. 2, and the curve marked "b" shows the curve produced
where the permanent magnets are positioned on the outer surface on
the end plates of the pot as shown on the left-hand side of FIG. 2.
The curve identified with "c" shows the curve of the radial
gradient which is produced if the permanent magnets are divided
between being positioned on the outer surface and being positioned
in the gap in the median plane in accordance with FIG. 3a. The
numerical ratio between the magnets is 6:8 in the curve shown in
curve 3c, i.e. 6 magnets were arranged in the centre and 8 on the
end plates. In this case, with a gap between the pole shoes of 18
cm and a pole shoe diameter of 40 cm, the homogeneity limit which
is represented by the dotted band 400 achieves a value of
G.sub.r=1.5.times.10.sup.-3 with r approximately 13 cm, more
preferably 12 cm. This limit 400 is displayed over the entire
height of the pot magnet, so that a usable transport volume of more
than 6 litres, e.g. more than 8 litres is provided within the pot
magnet, in which the homogeneity condition
G.sub.r.ltoreq.1.5.times.10.sup.-3/cm is fulfilled.
[0071] FIG. 6 shows a measurement record of the relaxation of the
.sup.3He [a] polarization in a storage cell of glass with a low
iron content. The volume of the storage cell is 350 cm.sup.3, the
gas pressure 2.5 bars. As can be seen from this figure, a
relaxation time of more than 70 hours is measured through the use
of such glasses, whereby the gradient-dependent relaxation time
could be ignored under the conditions for this measurement. If one
introduces such a receptacle consisting of glass with a low iron
content into the pot magnet in the region of the homogenised field,
a resulting total relaxation time T.sub.res=(.sub.1/T.sub.1
g+1/T.sub.1.sup.w).sup.-1 of 64 hours is achieved, based on a
gradient-dependent relaxation time of T.sub.1.sup.g=750 h and a
wall-related relaxation time of [T.sub.1.sup.g]T.sub.1.sup.w=70
h.
[0072] The method of the invention for removing a gas stored in a
storage cell 20 of a transport device in accordance with the
invention in the vicinity of an external magnetic field, for
example the stray field B.sub.TS of a nuclear magnetic resonance
imaging apparatus, is represented in FIGS. 7a and b. If the storage
cell is to be introduced into the field B.sub.T of the magnetic
resonance imaging apparatus, for a medical application for
instance, without this involving significant depolarization, the
invention proposes, as illustrated in FIG. 7a, that the transport
device in accordance with the invention be set up with its field
B.sub.o parallel to and in the same direction as the external
magnetic field B.sub.TS, as shown. The upper part of the transport
device facing the magnetic resonance imaging apparatus with the
pole shoe 10.1 is then lifted off in the direction indicated by the
arrow 302. This makes the storage cell 20 freely accessible. The
transport device, designed here in the form of a pot magnet, is
shown in its opened state in FIG. 7b. As can clearly be seen, the
homogenising force is reduced due to the upper section of the pot
magnet not being present. Nonetheless, the remaining lower pole
shoe 10.2 ensures that the magnetic field lines of the resulting
field B.sub.res end perpendicular on this pole shoe. This still
makes it possible to homogenise the magnetic field B.sub.res
adequately in the area of the storage cell 20, i.e. to achieve
parallel lines of magnetic force, as shown in the drawing. The
storage cell can then be removed along arrow 304 in the direction
of the symmetrical axis, in the field B.sub.res which is still
largely homogeneous even with the upper section removed, without a
noticeable depolarization of the gas occurring during the brief
time taken for removal.
[0073] Referring to FIG. 8 there is shown, in perspective, a
container according to the invention with hexagonal-cylindrical,
rather than circular cylindrical symmetry. Container 1 comprises a
hexagonal-cylindrical yoke 2 and has separable upper 1.1 and lower
1.2 portions. Magnetic field sources, pole shoes, etc. may be
disposed, e.g. as described for the variants described above, if
necessary including shims to combat edge effects to field
B.sub.o.
[0074] The gas contained in the storage cell designed in accordance
with the invented method still possesses an adequate degree of
polarization for the intended applications after being removed
within the strong magnetic field of the nuclear magnetic resonance
imaging apparatus.
[0075] This invention thus provides a device which allows the
storage and transport of spin polarized gases over long distances
and periods, such as is required in particular for an intended use
in the field of medicine. In particular, the invention is
characterised by its economical construction, simple design,
maximum possible useable volume and very low weight, whereby
reliable shielding against external stray fields is provided. The
invention thus provides, for the first time, a means which makes
the commercial use of .sup.3He and .sup.129Xe feasible, in the
field of medicine for example.
[0076] Regarding future possible uses of .sup.3He and .sup.129Xe in
medicine, particular reference is made to the use of polarized
.sup.3He and .sup.129Xe in brilliant, high-resolution,
three-dimensional nuclear magnetic resonance imaging of the human
respiratory system.
[0077] Regarding this application, reference is made to the
following publications, the disclosed content of which is included
in full in this application:
[0078] Bachert et al., Magnetic Resonance in Medicine 36: 192-196
(1996); and
[0079] Ebert et al., THE LANCET 347: 1297-1299 (1996).
[0080] In addition, a compact magnet of lightweight construction is
presented which provides a magnetic field which is both homogeneous
over a wide area, compact, easily transportable and relatively low
in cost and which, in particular, also fulfils all requirements in
terms of shielding off external magnetic fields which can lead to a
depolarization of the nuclear spin. The use of
commercially-available small permanent magnets represents a quite
decisive advantage in terms of both construction and economy.
[0081] In addition, there is the extremely high permeability and
low remanence of the .mu.-metal which is in this case used for the
first time for the construction of very thin, therefore lighter,
and yet highly-efficient pole shoes for the homogenisation of the
magnetic field.
[0082] The low magnetic flux also allows the use of a yoke made of
thin soft iron sheet which, at the same time, due to the pot form
and the associated possibility of radial conduction, adequately
shields off external interference fields.
[0083] This means that, in this invention, a magnet with an
extremely favourable ratio of homogeneous field volume to total
volume and very low weight is made available for the first
time.
[0084] In a slightly inferior construction form, pole shoes of
magnetically soft iron can be used in place of the .mu.-metal pole
shoes which, while reducing the quality of the field, represents a
more economical variant in terms of price. It is also possible to
replace the permanent magnets with magnetic field coils which
fulfil the same function, in order to generate the necessary flux
at the points required within the pot magnet.
[0085] Finally, a method for removing a spin polarized gas from the
pot unit in accordance with the invention is described in which the
degree of polarization is also maintained in the presence of
external magnetic fields, for example those of a nuclear magnetic
resonance imaging apparatus.
* * * * *